1. Field of Technology
The present invention relates to an in-mold coating method and apparatus for forming a molded product and subsequently forming a coating layer on one surface of the molded product both performed in one mold assembly.
2. Description of the Prior Art
The use of fiber-reinforced plastics in an automobile body structure is not a new development, and sheet molding compounds are examples of those plastics. The sheet molding compound is, as the nomenclature indicates, in the form of a sheet containing a fiber-reinforced thermosetting resin and a catalyst which serves as a cure-accelerating agent. When using this matter, one or more sheets of sheet molding compounds (SMC) are placed in a molding cavity defined between a pair of relatively movable dies forming the mold assembly and are then compressed together. Since the dies are heated to a temperature sufficient to fluidize the SMC sheets, the SMC sheets inside the molding cavity are fluidized in the molding cavity and then cured by the action of the catalyst contained therein to provide a molded article complemental in shape and size to the molding cavity.
Since the SMC sheet often contains air in its structure and/or since air tends to admix the SMC sheets when the latter are fluidized, it often occurs that the resultant molded article identified by A in FIG. 11 of the accompanying drawings contain cavities B as shown therein. Because of this, when a coating material or paint is applied on one surface of the molded article A and then baked to form a coating layer C, some of the cavities B in the molded article A adjacent the coating layer C blow up under the influence of heat used during the baking, resulting in the formation of pinholes or blow holes as indicated by D.
Once the pinholes or blow holes D are formed, surface imperfections are formed and, therefore the final product having such surface imperfections can no longer be used as a material for, for example, automobile door panels.
An in-mold coating technique such as disclosed in U.S. Pat. Nos. 4,076,788, issued Feb. 28, 1978; 4,235,833, issued Nov. 25, 1980; 4,245,976, issued Jan. 20, 1981; and 4,329,134, issued May 11, 1982 is considered effective to obviate the above discussed problem. According to the in-mold coating technique, both the compression molding and the coating are carried out in succession in a single mold assembly. FIG. 12 of the accompanying drawings is a reproduction, taken from the last three of the above mentioned U.S. patents, showing that portion of the molding assembly where a valved injector H is installed.
As shown in FIG. 12, the mold assembly comprises an upper die F having a molding surface which defines an outer surface of the eventually formed door panel, and a lower die G having a molding surface which defines an inner surface of the same door panel. When the upper and lower dies F and G are closed together, a molding cavity is formed between the molding surfaces of these dies, in which cavity is formed a molded product E. Subsequent to the compression molding of the molded product E, the upper die F is separated, while the mold assembly is still maintained in a sealed condition, to form a coating space between the molding surface of the upper die F and the molded product E. Thereafter, a controlled quantity of coating material is injected by the injector H into the coating space, followed by the movement of the upper die F close towards the lower die G to spread the injected coating material over the entire surface of the molded product E. The coating material so applied over the molded product E is subsequently cured to form a coating layer.
According to the in-mold coating technique, even though the molded product E has blow holes formed on one surface thereof confronting the coating space, the coating material so injected can enter and fill up such blow holes and, therefore, the aesthetically beautiful skin can be formed on the molded and coated product.
However, the in-mold coating technique utilizing the mold assembly such as shown in FIG. 12 has drawback in that, since the coating material is injected onto one surface of the molded product E in a direction confronting the molded product E, an outer surface of the resultant coating layer opposite the molded product E tends to have a marking left by a shut-off pin I which forms a valve member of the injector H, with the consequence that the aesthetic value of the molded and coated product is lowered.
The above described problem inherent in the prior art in-mold coating technique may be obviated if the molded product J is provided with a runner K which would be removed at the final stage of make-up of the molded and coated product as shown in FIG. 13. If the molded product J is provided with the runner K and the shut-off pin L of the injector is so positioned as to align with this runner K, no marking left by the shut-off pin L is seen in the molded and coated product because the runner is removed. However, this contemplated method is problematic in that the coating material when injected and subsequently compressed can not always uniformly spread over the entire surface of the molded product, resulting in the formation of the coating layer of irregular thickness. The thickness of the coating layer at a location furthest from the position where the runner had been formed would be smaller than that at a location close to that position.
The increased number of the runners and the use of the correspondingly increased number of the injectors, one for each runner, would obviate the above described drawback, however, not only does the system as a whole become bulky; but also the increased manufacturing cost will be incurred.